Establishing Correlates of Protection for Vaccine Development: Considerations for the Respiratory Syncytial Virus Vaccine Field

Current state and challenges in respiratory syncytial virus drug discovery and development

Human respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infections (LRTI) in young children and elderly people worldwide. Recent significant progress in our understanding of the structure and function of RSV proteins has led to the discovery of several clinical candidates targeting RSV fusion and replication. These include both the development of novel small molecule interventions and the isolation of potent monoclonal antibodies. In this review, we summarize the state-of-the-art of RSV drug discovery, with a focus on the characteristics of the candidates that reached the clinical stage of development. We also discuss the lessons learned from failed and discontinued clinical developments and highlight the challenges that remain for development of RSV therapies.

Vaccine-induced antibody Fc-effector functions in humans immunized with a combination Ad26.RSV.preF/RSV preF protein vaccine

IMPORTANCE

Respiratory syncytial virus (RSV) can cause serious illness in older adults (i.e., those aged ≥60 years). Because options for RSV prophylaxis and treatment are limited, the prevention of RSV-mediated illness in older adults remains an important unmet medical need. Data from prior studies suggest that Fc-effector functions are important for protection against RSV infection. In this work, we show that the investigational Ad26.RSV.preF/RSV preF protein vaccine induced Fc-effector functional immune responses in adults aged ≥60 years who were enrolled in a phase 1/2a regimen selection study of Ad26.RSV.preF/RSV preF protein. These results demonstrate the breadth of the immune responses induced by the Ad26.RSV.preF/RSV preF protein vaccine.

Value profile for respiratory syncytial virus vaccines and monoclonal antibodies

Respiratory syncytial virus (RSV) is the predominant cause of acute lower respiratory infection (ALRI) in young children worldwide, yet no licensed RSV vaccine exists to help prevent the millions of illnesses and hospitalizations and tens of thousands of young lives taken each year. Monoclonal antibody (mAb) prophylaxis exists for prevention of RSV in a small subset of very high-risk infants and young children, but the only currently licensed product is impractical, requiring multiple doses and expensive for the low-income settings where the RSV disease burden is greatest. A robust candidate pipeline exists to one day prevent RSV disease in infant and pediatric populations, and it focuses on two promising passive immunization approaches appropriate for low-income contexts: maternal RSV vaccines and long-acting infant mAbs. Licensure of one or more candidates is feasible over the next one to three years and, depending on final product characteristics, current economic models suggest both approaches are likely to be cost-effective. Strong coordination between maternal and child health programs and the Expanded Program on Immunization will be needed for effective, efficient, and equitable delivery of either intervention. This 'Vaccine Value Profile' (VVP) for RSV is intended to provide a high-level, holistic assessment of the information and data that are currently available to inform the potential public health, economic and societal value of pipeline vaccines and vaccine-like products. This VVP was developed by a working group of subject matter experts from academia, non-profit organizations, public private partnerships and multi-lateral organizations, and in collaboration with stakeholders from the WHO headquarters. All contributors have extensive expertise on various elements of the RSV VVP and collectively aimed to identify current research and knowledge gaps. The VVP was developed using only existing and publicly available information.

Fc-mediated functions of nirsevimab complement direct respiratory syncytial virus neutralization but are not required for optimal prophylactic protection

Introduction

Nirsevimab is an extended half-life (M252Y/S254T/T256E [YTE]-modified) monoclonal antibody to the pre-fusion conformation of the respiratory syncytial virus (RSV) Fusion protein, with established efficacy in preventing RSV-associated lower respiratory tract infection in infants for the duration of a typical RSV season. Previous studies suggest that nirsevimab confers protection via direct virus neutralization. Here we use preclinical models to explore whether fragment crystallizable (Fc)-mediated effector functions contribute to nirsevimab-mediated protection.

Methods

Nirsevimab, MEDI8897* (i.e., nirsevimab without the YTE modification), and MEDI8897*-TM (i.e., MEDI8897* without Fc effector functions) binding to Fc γ receptors (FcγRs) was evaluated using surface plasmon resonance. Antibody-dependent neutrophil phagocytosis (ADNP), antibody-dependent cellular phagocytosis (ADCP), antibody-dependent complement deposition (ADCD), and antibody-dependent cellular cytotoxicity (ADCC) were assessed through in vitro and ex vivo serological analyses. A cotton rat challenge study was performed with MEDI8897* and MEDI8897*-TM to explore whether Fc effector functions contribute to protection from RSV.

Results

Nirsevimab and MEDI8897* exhibited binding to a range of FcγRs, with expected reductions in FcγR binding affinities observed for MEDI8897*-TM. Nirsevimab exhibited in vitro ADNP, ADCP, ADCD, and ADCC activity above background levels, and similar ADNP, ADCP, and ADCD activity to palivizumab. Nirsevimab administration increased ex vivo ADNP, ADCP, and ADCD activity in participant serum from the MELODY study (NCT03979313). However, ADCC levels remained similar between nirsevimab and placebo. MEDI8897* and MEDI8897*-TM exhibited similar dose-dependent reduction in lung and nasal turbinate RSV titers in the cotton rat model.

Conclusion

Nirsevimab possesses Fc effector activity comparable with the current standard of care, palivizumab. However, despite possessing the capacity for Fc effector activity, data from RSV challenge experiments illustrate that nirsevimab-mediated protection is primarily dependent on direct virus neutralization.

Combination Ad26.RSV.preF/preF protein vaccine induces superior protective immunity compared with individual vaccine components in preclinical models

Respiratory syncytial virus (RSV) is a leading cause of severe respiratory disease for which no licensed vaccine is available. We have previously shown that a prefusion (preF) conformation-stabilized RSV F protein antigen and an adenoviral vector encoding RSV preF protein (Ad26.RSV.preF) are immunogenic and protective in animals when administered as single components. Here, we evaluated a combination of the 2 components, administered as a single injection. Strong induction of both humoral and cellular responses was shown in RSV-naïve and pre-exposed mice and pre-exposed African green monkeys (AGMs). Both components of the combination vaccine contributed to humoral immune responses, while the Ad26.RSV.preF component was the main contributor to cellular immune responses in both mice and AGMs. Immunization with the combination elicited superior protection against RSV A2 challenge in cotton rats. These results demonstrate the advantage of a combination vaccine and support further clinical development.

Recent updates on correlates of vaccine-induced protection

Correlates of protection are key for vaccine development against any pathogen. In this paper we summarize recent information about correlates for vaccines against dengue, Ebola, influenza, pneumococcal, respiratory syncytial virus, rotavirus, shigella, tuberculosis and Zika virus.

Antibody effector functions are associated with protection from respiratory syncytial virus

Respiratory syncytial virus (RSV) infection is a major cause of severe lower respiratory tract infection and death in young infants and the elderly. With no effective prophylactic treatment available, current vaccine candidates aim to elicit neutralizing antibodies. However, binding and neutralization have poorly predicted protection in the past, and accumulating data across epidemiologic cohorts and animal models collectively point to a role for additional antibody Fc-effector functions. To begin to define the humoral correlates of immunity against RSV, here we profiled an adenovirus 26 RSV-preF vaccine-induced humoral immune response in a group of healthy adults that were ultimately challenged with RSV. Protection from infection was linked to opsonophagocytic functions, driven by IgA and differentially glycosylated RSV-specific IgG profiles, marking a functional humoral immune signature of protection against RSV. Furthermore, Fc-modified monoclonal antibodies able to selectively recruit effector functions demonstrated significant antiviral control in a murine model of RSV.

Virus reduction neutralization test and LI-COR microneutralization assay bridging and WHO international standard calibration studies for respiratory syncytial virus

Background: Respiratory syncytial virus (RSV) vaccine is an unmet medical need. The virus reduction neutralization test (VRNT) was developed to replace the LI-COR microneutralization assay to measure RSV neutralization titers. Methods: A bridging study using selected V171 phase I samples and calibration studies using the WHO international standard antiserum to RSV were performed to compare VRNT and LI-COR. Results: From the bridging study, we showed good concordance between VRNT and LI-COR titers, and similar post-/pre-vaccination titer ratios. From the calibration studies, we can convert VRNT and LI-COR titers into similar IU/ml. Conclusion: The VRNT and LI-COR microneutralization assay correlate well and the titers can be standardized as similar IU/ml, enabling direct comparison of titers from different assays.

Longitudinal study of the immune response and memory following natural bovine respiratory syncytial virus infections in cattle of different age

Human and bovine respiratory syncytial virus (HRSV and BRSV) are closely genetically related and cause respiratory disease in their respective host. Whereas HRSV vaccines are still under development, a multitude of BRSV vaccines are used to reduce clinical signs. To enable the design of vaccination protocols to entirely stop virus circulation, we aimed to investigate the duration, character and efficacy of the immune responses induced by natural infections. The systemic humoral immunity was monitored every two months during two years in 33 dairy cattle in different age cohorts following a natural BRSV outbreak, and again in selected individuals before and after a second outbreak, four years later. Local humoral and systemic cellular responses were also monitored, although less extensively. Based on clinical observations and economic losses linked to decreased milk production, the outbreaks were classified as moderate. Following the first outbreak, most but not all animals developed neutralising antibody responses, BRSV-specific IgG1, IgG2 and HRSV F- and HRSV N-reactive responses that lasted at least two years, and in some cases at least four years. In contrast, no systemic T cell responses were detected and only weak IgA responses were detected in some animals. Seronegative sentinels remained negative, inferring that no new infections occurred between the outbreaks. During the second outbreak, reinfections with clinical signs and virus shedding occurred, but the signs were milder, and the virus shedding was significantly lower than in naïve animals. Whereas the primary infection induced similar antibody titres against the prefusion and the post fusion form of the BRSV F protein, memory responses were significantly stronger against prefusion F. In conclusion, even if natural infections induce a long-lasting immunity, it would probably be necessary to boost memory responses between outbreaks, to stop the circulation of the virus and limit the potential role of previously infected adult cattle in the chain of BRSV transmission.

RSV disease in infants and young children: Can we see a brighter future?

Respiratory syncytial virus (RSV) is a highly contagious seasonal virus and the leading cause of Lower Respiratory Tract Infections (LRTI), including pneumonia and bronchiolitis in children. RSV-related LRTI cause approximately 3 million hospitalizations and 120,000 deaths annually among children <5 years of age. The majority of the burden of RSV occurs in previously healthy infants. Only a monoclonal antibody (mAb) has been approved against RSV infections in a restricted group, leaving an urgent unmet need for a large number of children potentially benefiting from preventive measures. Approaches under development include maternal vaccines to protect newborns, extended half-life monoclonal antibodies to provide rapid long-lasting protection, and pediatric vaccines. RSV has been identified as a major global priority but a solution to tackle this unmet need for all children has yet to be implemented. New technologies represent the avenue for effectively addressing the leading-cause of hospitalization in children <1 years old.

Immunopathology of RSV: An Updated Review

RSV is a leading cause of respiratory tract disease in infants and the elderly. RSV has limited therapeutic interventions and no FDA-approved vaccine. Gaps in our understanding of virus-host interactions and immunity contribute to the lack of biological countermeasures. This review updates the current understanding of RSV immunity and immunopathology with a focus on interferon responses, animal modeling, and correlates of protection.

Suptavumab for the Prevention of Medically Attended Respiratory Syncytial Virus Infection in Preterm Infants

BACKGROUND

Respiratory syncytial virus (RSV) is a major cause of childhood medically attended respiratory infection (MARI).

METHODS

We conducted a randomized, double-blind, placebo-controlled phase 3 trial in 1154 preterm infants of 1 or 2 doses of suptavumab, a human monoclonal antibody that can bind and block a conserved epitope on RSV A and B subtypes, for the prevention of RSV MARI. The primary endpoint was proportion of subjects with RSV-confirmed hospitalizations or outpatient lower respiratory tract infection (LRTI).

RESULTS

There were no significant differences between primary endpoint rates (8.1%, placebo; 7.7%, 1-dose; 9.3%, 2-dose). Suptavumab prevented RSV A infections (relative risks, .38; 95% confidence interval [CI], .14-1.05 in the 1-dose group and .39 [95% CI, .14-1.07] in the 2-dose group; nominal significance of combined suptavumab group vs placebo; P = .0499), while increasing the rate of RSV B infections (relative risk 1.36 [95% CI, .73-2.56] in the 1-dose group and 1.69 [95% CI, .92-3.08] in the 2-dose group; nominal significance of combined suptavumab group vs placebo; P = .12). Sequenced RSV isolates demonstrated no suptavumab epitope changes in RSV A isolates, while all RSV B isolates had 2-amino acid substitution in the suptavumab epitope that led to loss of neutralization activity. Treatment emergent adverse events were balanced across treatment groups.

CONCLUSIONS

Suptavumab did not reduce overall RSV hospitalizations or outpatient LRTI because of a newly circulating mutant strain of RSV B. Genetic variation in circulating RSV strains will continue to challenge prevention efforts.

CLINICAL TRIALS REGISTRATION

NCT02325791.

Forward and reverse translational approaches to predict efficacy of neutralizing respiratory syncytial virus (RSV) antibody prophylaxis

BACKGROUND

Neutralizing mAbs can prevent communicable viral diseases. MK-1654 is a respiratory syncytial virus (RSV) F glycoprotein neutralizing monoclonal antibody (mAb) under development to prevent RSV infection in infants. Development and validation of methods to predict efficacious doses of neutralizing antibodies across patient populations exposed to a time-varying force of infection (i.e., seasonal variation) are necessary.

METHODS

Five decades of clinical trial literature were leveraged to build a model-based meta-analysis (MBMA) describing the relationship between RSV serum neutralizing activity (SNA) and clinical endpoints. The MBMA was validated by backward translation to animal challenge experiments and forward translation to predict results of a recent RSV mAb trial. MBMA predictions were evaluated against a human trial of 70 participants who received either placebo or one of four dose-levels of MK-1654 and were challenged with RSV [NCT04086472]. The MBMA was used to perform clinical trial simulations and predict efficacy of MK-1654 in the infant target population.

FINDINGS

The MBMA established a quantitative relationship between RSV SNA and clinical endpoints. This relationship was quantitatively consistent with animal model challenge experiments and results of a recently published clinical trial. Additionally, SNA elicited by increasing doses of MK-1654 in humans reduced RSV symptomatic infection rates with a quantitative relationship that approximated the MBMA. The MBMA indicated a high probability that a single dose of ≥ 75 mg of MK-1654 will result in prophylactic efficacy (> 75% for 5 months) in infants.

INTERPRETATION

An MBMA approach can predict efficacy of neutralizing antibodies against RSV and potentially other respiratory pathogens.

Antibody Persistence 6 Months Post-Vaccination with BNT162b2 among Health Care Workers

BACKGROUND

We present immunogenicity data 6 months after the first dose of BNT162b2 in correlation with age, gender, BMI, comorbidities and previous SARS-CoV-2 infection.

METHODS

An immunogenicity evaluation was carried out among health care workers (HCW) vaccinated at the Istituti Fisioterapici Ospitalieri (IFO). All HCW were asked to be vaccine by the national vaccine campaign at the beginning of 2021. Serum samples were collected on day 1 just prior to the first dose of the vaccine and on day 21 just prior to the second vaccination dose. Thereafter sera samples were collected 28, 49, 84 and 168 days after the first dose of BNT162b2. Quantitative measurement of IgG antibodies against S1/S2 antigens of SARS-CoV-2 was performed with a commercial chemiluminescent immunoassay.

RESULTS

Two hundred seventy-four HWCs were analyzed, 175 women (63.9%) and 99 men (36.1%). The maximum antibody geometric mean concentration (AbGMC) was reached at T2 (299.89 AU/mL; 95% CI: 263.53-339.52) with a significant increase compared to baseline (p < 0.0001). Thereafter, a progressive decrease was observed. At T5, a median decrease of 59.6% in COVID-19 negative, and of 67.8% in COVID-19 positive individuals were identified with respect to the highest antibody response. At T1, age and previous COVID-19 were associated with differences in antibody response, while at T2 and T3 differences in immune response were associated with age, gender and previous COVID-19. At T4 and T5, only COVID-19 positive participants demonstrated a greater antibody response, whereas no other variables seemed to influence antibody levels.

CONCLUSIONS

Overall our study clearly shows antibody persistence at 6 months, albeit with a certain decline. Thus, the use of this vaccine in addressing the COVID-19 pandemic is supported by our results that in turn open debate about the need for further boosts.

Immunogenicity and Safety of COVID-19 Vaccine BNT162b2 for Patients with Solid Cancer: A Large Cohort Prospective Study from a Single Institution

PURPOSE

We assessed the immunogenicity and safety of the BNT162b2 vaccine in a large cohort of patients with cancer (CP).

EXPERIMENTAL DESIGN

From March 1, 2021 to March 20, 2021, this prospective cohort study included 816 CP afferent to our institution and eligible for the vaccination. A cohort of 274 health care workers (HCW) was used as age- and sex-matched control group. BNT162b2 was administered as a two-dose regimen given 21 days apart. Blood samples to analyze anti-Spike (S) IgG antibodies (Ab) were collected prevaccination [timepoint (TP) 0], and at 3 weeks (TP1) and 7 weeks (TP2) after the first dose.

RESULTS

Patients characteristics: median age 62 (range, 21-97); breast/lung cancer/others (31/21/48%); active treatment/follow-up (90/10%). In the whole CP cohort, the serologic response rate (RR) and the titre of anti-S IgG significantly increased across the TPs; at TP2, the responders (IgG >15 AU/mL) were 94.2%. Active chemotherapy and chronic use of steroids were independent predictors of lower RR. Adverse events (AE) after the booster predicted higher likelihood of response (OR, 4.04; 95% confidence interval, 1.63-9.99; P = 0.003). Comparing the matched cohorts, the responders were significantly lower in CP than in HCW at TP1 (61.2% vs. 93.2%) and TP2 (93.3% vs. 100%), while the geometric mean concentration of IgG did not significantly differ at TP2 being significantly lower in CP (23.3) than in HCW (52.1) at TP1. BNT162b2 was well tolerated in CP; severe-grade AEs were 3.5% and 1.3% after the first and second doses, respectively.

CONCLUSIONS

BNT162b2 assures serologic immunization without clinically significant toxicity in CP. The second dose is needed to reach a satisfactory humoral response.

B cell genomics behind cross-neutralization of SARS-CoV-2 variants and SARS-CoV

Monoclonal antibodies (mAbs) are a focus in vaccine and therapeutic design to counteract severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its variants. Here, we combined B cell sorting with single-cell VDJ and RNA sequencing (RNA-seq) and mAb structures to characterize B cell responses against SARS-CoV-2. We show that the SARS-CoV-2-specific B cell repertoire consists of transcriptionally distinct B cell populations with cells producing potently neutralizing antibodies (nAbs) localized in two clusters that resemble memory and activated B cells. Cryo-electron microscopy structures of selected nAbs from these two clusters complexed with SARS-CoV-2 spike trimers show recognition of various receptor-binding domain (RBD) epitopes. One of these mAbs, BG10-19, locks the spike trimer in a closed conformation to potently neutralize SARS-CoV-2, the recently arising mutants B.1.1.7 and B.1.351, and SARS-CoV and cross-reacts with heterologous RBDs. Together, our results characterize transcriptional differences among SARS-CoV-2-specific B cells and uncover cross-neutralizing Ab targets that will inform immunogen and therapeutic design against coronaviruses.

Respiratory syntycial virus: Current treatment strategies and vaccine approaches

Respiratory Syncytial Virus is a yearly respiratory virus that causes significant frequencies of morbidities, particularly in the young and elderly populations. However, preventive vaccines and/or treatment therapies are generally lacking, although much attention is now being placed on this virus. Moreover, there are now multiple strategies currently being explored in a race to the first licensed vaccine. While vaccines are being developed, multiple treatment strategies are being explored to attenuate the severity of infection and thus reduce hospitalization rates in vulnerable populations. This review outlines current strategies to prevent or treat this virus in the hopes of reducing significant human morbidity and mortality that occurs yearly with this seasonal virus.

Initial observations on age, gender, BMI and hypertension in antibody responses to SARS-CoV-2 BNT162b2 vaccine

BACKGROUND

Literature data suggests that age, gender and body mass index (BMI) could be associated with difference in immune responses to vaccines. The first goal of the study was to analyze the antibody titre seven days after the second dose of BNT162b2 vaccine in a group of 248 healthcare workers (HCWs). The second goal was to analyze how antibody titre changes in correlation with age, gender, BMI and hypertension.

METHODS

An immunogenicity evaluation was carried out among HCWs vaccinated at the Istituti Fisioterapici Ospitalieri (IFO), Rome, Italy. All HCWs were asked to be vaccinated by the Italian national vaccine campaign at the beginning of 2021. 260 vaccinated HCWs were enrolled in the study. All eligible participants were assigned to receive the priming dose in two weeks' time and the booster dose exactly 21 days thereafter. Blood and nasopharyngeal swabs were collected at baseline and 7 days after second dose of vaccine. Quantitative measurements of IgG antibodies against S1/S2 antigens of SARS-CoV-2 were performed with a commercial chemiluminescent immunoassay. Presence of SARS-Cov-2 in nasopharyngeal swab was determined by commercial RT-PCR testing.

FINDINGS

248 HWCs were analyzed, 158 women (63.7%) and 90 men (36.3%). After the second dose of BNT162b2 vaccine, 99.5% of participants developed a humoral immune response. The geometric mean concentration of antibodies among the vaccinated subjects after booster dose (285.9 AU/mL 95% CI: 249.5-327.7) was higher than that of human convalescent sera (39.4 AU/mL, 95% CI: 33.1-46.9), with p<0.0001. Multivariate linear regression analysis of AU/mL by age, gender and BMI multivariate was performed by the inclusion of covariates. This analysis demonstrated that age (p<0.0001) and gender (p = 0.038) are statistically associated with differences in antibody response after vaccination, whereas BMI and hypertension have no statistically significant association (p = 0.078 and p = 0.52 respectively).

INTERPRETATION

99.5% of HCW developed a humoral immune response and female and young participants seem to have an increased capacity to mount humoral immune responses. BMI and hypertension seem not associated with difference in immune response to the vaccine.

FUNDING

None.

Humoral and Mucosal Antibody Response to RSV Structural Proteins in RSV-Infected Adult Hematopoietic Cell Transplant (HCT) Recipients

Respiratory syncytial virus (RSV) is an important cause of lower respiratory tract infection in infants, the elderly, and immunocompromised patients. RSV antibodies play a role in preventing reinfection and in clearance of RSV, but data regarding the levels of viral protein-specific antibodies elicited and their contribution to patient recovery from RSV-induced disease are limited. We prospectively enrolled a cohort of RSV-infected adult hematopoietic cell transplant (HCT) recipients (n = 40). Serum and nasal-wash samples were obtained at enrollment (acute samples) and convalescence (convalescent samples). We measured (1) humoral IgG and mucosal IgA binding antibody levels to multiple RSV proteins (F, G, N, P, and M2-1) by Western blot (WB); (2) neutralizing antibody (Nt Ab) titers by microneutralization assay; and (3) palivizumab-like antibody (PLA) concentrations by an ELISA-based competitive binding assay developed in the lab. Finally, we tested for correlations between protein-specific antibody levels and duration of viral shedding (normal: cleared in <14 days and delayed: cleared ≥14 days), as well as RSV/A and RSV/B subtypes. Convalescent sera from HCT recipients had significantly higher levels of anti-RSV antibodies to all 5 RSV structural proteins assayed (G, F, N, P, M2-1), higher Nt Abs to both RSV subtypes, and higher serum PLAs than at enrollment. Significantly higher levels of mucosal antibodies to 3 RSV structural proteins (G, N, and M2-1) were observed in the convalescent nasal wash versus acute nasal wash. Normal viral clearance group had significantly higher levels of serum IgG antibodies to F, N, and P viral proteins, higher Nt Ab to both RSV subtypes, and higher PLA, as well as higher levels of mucosal IgA antibodies to G and M2-1 viral proteins, and higher Nt Ab to both RSV subtypes compared to delayed viral clearance group. Normal RSV clearance was associated with higher IgG serum antibody levels to F and P viral proteins, and PLAs in convalescent serum (p < 0.05). Finally, overall antibody levels in RSV/A- and/B-infected HCT recipients were not significantly different. In summary, specific humoral and mucosal RSV antibodies are associated with viral clearance in HCT recipients naturally infected with RSV. In contrast to the humoral response, the F surface glycoprotein was not a major target of mucosal immunity. Our findings have implications for antigen selection in the development of RSV vaccines.

Sensitivity of SARS-CoV-2 B.1.1.7 to mRNA vaccine-elicited antibodies

Transmission of SARS-CoV-2 is uncontrolled in many parts of the world; control is compounded in some areas by the higher transmission potential of the B.1.1.7 variant1, which has now been reported in 94 countries. It is unclear whether the response of the virus to vaccines against SARS-CoV-2 on the basis of the prototypic strain will be affected by the mutations found in B.1.1.7. Here we assess the immune responses of individuals after vaccination with the mRNA-based vaccine BNT162b22. We measured neutralizing antibody responses after the first and second immunizations using pseudoviruses that expressed the wild-type spike protein or a mutated spike protein that contained the eight amino acid changes found in the B.1.1.7 variant. The sera from individuals who received the vaccine exhibited a broad range of neutralizing titres against the wild-type pseudoviruses that were modestly reduced against the B.1.1.7 variant. This reduction was also evident in sera from some patients who had recovered from COVID-19. Decreased neutralization of the B.1.1.7 variant was also observed for monoclonal antibodies that target the N-terminal domain (9 out of 10) and the receptor-binding motif (5 out of 31), but not for monoclonal antibodies that recognize the receptor-binding domain that bind outside the receptor-binding motif. Introduction of the mutation that encodes the E484K substitution in the B.1.1.7 background to reflect a newly emerged variant of concern (VOC 202102/02) led to a more-substantial loss of neutralizing activity by vaccine-elicited antibodies and monoclonal antibodies (19 out of 31) compared with the loss of neutralizing activity conferred by the mutations in B.1.1.7 alone. The emergence of the E484K substitution in a B.1.1.7 background represents a threat to the efficacy of the BNT162b2 vaccine.

SARS-CoV-2 B.1.1.7 sensitivity to mRNA vaccine-elicited, convalescent and monoclonal antibodies

Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) transmission is uncontrolled in many parts of the world, compounded in some areas by higher transmission potential of the B1.1.7 variant now seen in 50 countries. It is unclear whether responses to SARS-CoV-2 vaccines based on the prototypic strain will be impacted by mutations found in B.1.1.7. Here we assessed immune responses following vaccination with mRNA-based vaccine BNT162b2. We measured neutralising antibody responses following a single immunization using pseudoviruses expressing the wild-type Spike protein or the 8 amino acid mutations found in the B.1.1.7 spike protein. The vaccine sera exhibited a broad range of neutralising titres against the wild-type pseudoviruses that were modestly reduced against B.1.1.7 variant. This reduction was also evident in sera from some convalescent patients. Decreased B.1.1.7 neutralisation was also observed with monoclonal antibodies targeting the N-terminal domain (9 out of 10), the Receptor Binding Motif (RBM) (5 out of 31), but not in neutralising mAbs binding outside the RBM. Introduction of the E484K mutation in a B.1.1.7 background to reflect newly emerging viruses in the UK led to a more substantial loss of neutralising activity by vaccine-elicited antibodies and mAbs (19 out of 31) over that conferred by the B.1.1.7 mutations alone. E484K emergence on a B.1.1.7 background represents a threat to the vaccine BNT162b.

An mRNA Vaccine against SARS-CoV-2 - Preliminary Report

BACKGROUND

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged in late 2019 and spread globally, prompting an international effort to accelerate development of a vaccine. The candidate vaccine mRNA-1273 encodes the stabilized prefusion SARS-CoV-2 spike protein.

METHODS

We conducted a phase 1, dose-escalation, open-label trial including 45 healthy adults, 18 to 55 years of age, who received two vaccinations, 28 days apart, with mRNA-1273 in a dose of 25 μg, 100 μg, or 250 μg. There were 15 participants in each dose group.

RESULTS

After the first vaccination, antibody responses were higher with higher dose (day 29 enzyme-linked immunosorbent assay anti-S-2P antibody geometric mean titer [GMT], 40,227 in the 25-μg group, 109,209 in the 100-μg group, and 213,526 in the 250-μg group). After the second vaccination, the titers increased (day 57 GMT, 299,751, 782,719, and 1,192,154, respectively). After the second vaccination, serum-neutralizing activity was detected by two methods in all participants evaluated, with values generally similar to those in the upper half of the distribution of a panel of control convalescent serum specimens. Solicited adverse events that occurred in more than half the participants included fatigue, chills, headache, myalgia, and pain at the injection site. Systemic adverse events were more common after the second vaccination, particularly with the highest dose, and three participants (21%) in the 250-μg dose group reported one or more severe adverse events.

CONCLUSIONS

The mRNA-1273 vaccine induced anti-SARS-CoV-2 immune responses in all participants, and no trial-limiting safety concerns were identified. These findings support further development of this vaccine. (Funded by the National Institute of Allergy and Infectious Diseases and others; mRNA-1273 ClinicalTrials.gov number, NCT04283461).

A Phase 1 Randomized, Double-Blind, Placebo-Controlled Trial to Assess the Safety, Tolerability, and Pharmacokinetics of a Respiratory Syncytial Virus Neutralizing Monoclonal Antibody MK-1654 in Healthy Adults

Respiratory syncytial virus (RSV) is the leading cause of acute lower respiratory tract infection and related morbidity and mortality in infants. Passive immunization with an RSV-neutralizing antibody can provide rapid protection to this vulnerable population. Proof-of-concept for this approach has been demonstrated by palivizumab; however, the use of this antibody is generally restricted to the highest-risk infants due to monthly dosing requirements and its cost. To address the large unmet medical need for most infants, we are evaluating MK-1654, a fully human RSV-neutralizing antibody with half-life extending mutations targeting site IV of the fusion protein. In this 2-part, placebo-controlled, double-blind, first-in-human study, 152 healthy adults were randomized 3:1 to receive a single dose of MK-1654 or placebo in 5 cohorts (100 or 300 mg as an intramuscular dose or 300, 1000, or 3000 mg as an intravenous dose). Safety, pharmacokinetics, antidrug antibodies, and RSV serum-neutralizing antibody titers were evaluated through 1 year. MK-1654 serum concentrations increased proportionally with dose and resulted in corresponding elevations in RSV serum-neutralizing antibody titers. The antibody displayed a half-life of 73 to 88 days and an estimated bioavailability of 69% at the 300-mg dose. The overall safety profile of MK-1654 was similar to placebo, and treatment-emergent antidrug antibodies were low (2.6%) with no associated adverse events. These data support the continued development of MK-1654 for the prevention of RSV disease in infants.

Discordance between Serum Neutralizing Antibody Titers and the Recovery from COVID-19

The recent pandemic of COVID-19 has caused a tremendous alarm around the world. Details of the infection process in the host have significant bearings on both recovery from the disease and on the correlates of the protection from the future exposures. One of these factors is the presence and titers of neutralizing Abs (NAbs) in infected people. In the current study, we set out to investigate NAbs in the recovered subjects discharged from the hospital in full health. Serum samples from a total of 49 documented consecutive COVID-19 subjects were included in the study. All the subjects were adults, and serum samples collected during the discharge were tested in viral neutralization, enzyme immunoassay (EIA), and Western immunoblot tests against viral Ags. Even though a majority of the recovered subjects had raised significant NAb titers, there is a substantial number of recovered patients (10 out of 49) with no or low titers of NAbs against the virus. In these cohorts as well as in patients with high NAb titers, viral Ag binding Abs were detectable in EIA tests. Both NAb titers and EIA detectable Abs are increased in patients experiencing a severe form of the disease, and in older patients the Ab titers were heightened. The main conclusion is that the recovery from SARS-CoV-2 infection is not solely dependent on high NAb titers in affected subjects, and this recovery process is probably produced by a complex interplay between many factors, including immune response, age of the subjects, and viral pathology.

Vaccine Design Informed by Virus-Induced Immunity

When an individual is exposed to a viral pathogen for the first time, the adaptive immune system is naive and cannot prevent virus replication. The consequence may be severe disease. At the same time, the host may rapidly generate a pathogen-specific immune response that will prevent disease if the virus is encountered again. Parvovirus B19 provides one such example. Children with sickle cell disease can experience life-threatening transient aplastic crisis when first exposed to parvovirus B19, but an effective immune response confers lifelong protection. We briefly examine the induction and benefits of virus-induced immunity. We focus on three human viruses for which there are no licensed vaccines (respiratory syncytial virus, human immunodeficiency virus type 1, and parvovirus B19) and consider how virus-induced immunity may inform successful vaccine design.

Updates on immunologic correlates of vaccine-induced protection

Correlates of protection (CoPs) are increasingly important in the development and licensure of vaccines. Although the study of CoPs was initially directed at identifying a single immune function that could explain vaccine efficacy, it has become increasingly clear that there are often multiple functions responsible for efficacy. This review is meant to supplement prior articles on the subject, illustrating both simple and complex CoPs.

Respiratory syncytial virus prefusogenic fusion (F) protein nanoparticle vaccine: Structure, antigenic profile, immunogenicity, and protection

Respiratory syncytial virus (RSV) is a major cause of severe respiratory disease in the very young, elderly, and immunocompromised for which there is no vaccine. The surface exposed RSV fusion (F) glycoprotein is required for membrane fusion and infection and is a desirable vaccine candidate. RSV F glycoprotein structure is dynamic and undergoes significant rearrangements during virus assembly, fusion, and infection. We have previously described an RSV fusion-inactive prefusogenic F with a mutation of one of two furin cleavage sites resulting in the p27 region on the N-terminus of F1 with a truncated fusion peptide covalently linked to F2. A processing intermediate RSV prefusogenic F has been reported in infected cells, purified F, budded virus, and elicited a strong immune response against p27 in RSV infected young children. In this report, we demonstrate that prefusogenic F, when expressed on the cell surface of Sf9 insect and human 293T cells, binds monoclonal antibodies (mAbs) that target prefusion-specific antigenic sites Ø and VIII, and mAbs targeting epitopes common to pre- and postfusion F sites II and IV. Purified prefusogenic F bound prefusion F specific mAbs to antigenic sites Ø and VIII and mAbs targeting pre- and postfusion sites II, IV, and p27. Mice immunized with prefusogenic F antigen produced significantly higher levels of anti-F IgG and RSV neutralizing antibodies than prefusion or postfusion F antigens and induced antibodies competitive with mAbs to sites Ø, VIII, II, and IV. RSV prefusogenic F neutralization antibody responses were enhanced with aluminum phosphate adjuvant and significantly higher than prefusion F. Prefusogenic F vaccine protected cotton rats against upper and lower respiratory tract infection by RSV/A. For the first time, we present the structure, antigenic profile, immunogenicity, and protective efficacy of RSV prefusogenic F nanoparticle vaccine.

RSV Vaccine Based on Rhabdoviral Vector Protects after Single Immunization

The respiratory syncytial virus (RSV) is one major cause of lower respiratory tract infections in childhood and an effective vaccine is still not available. We previously described a new rhabdoviral vector vaccine, VSV-GP, a variant of the vesicular stomatitis virus (VSV), where the VSV glycoprotein G is exchanged by the glycoprotein GP of the lymphocytic choriomeningitis virus. Here, we evaluated VSV-GP as vaccine vector for RSV with the aim to induce RSV neutralizing antibodies. Wild-type F (Fwt) or a codon optimized version (Fsyn) were introduced at position 5 into the VSV-GP genome. Both F versions were efficiently expressed in VSV-GP-F infected cells and incorporated into VSV-GP particles. In mice, high titers of RSV neutralizing antibodies were induced already after prime and subsequently boosted by a second immunization. After challenge with RSV, viral loads in the lungs of immunized mice were reduced by 2-3 logs with no signs of an enhanced disease induced by the vaccination. Even a single intranasal immunization significantly reduced viral load by a factor of more than 100-fold. RSV neutralizing antibodies were long lasting and mice were still protected when challenged 20 weeks after the boost. Therefore, VSV-GP is a promising candidate for an effective RSV vaccine.

Will Attention by Vaccine Developers to the Host's Nuclear Hormone Levels and Immunocompetence Improve Vaccine Success?

Despite extraordinary advances in fields of immunology and infectious diseases, vaccine development remains a challenge. The development of a respiratory syncytial virus vaccine, for example, has spanned more than 50 years of research with studies of more than 100 vaccine candidates. Dozens of attractive vaccine products have entered clinical trials, but none have completed the path to licensing. Human immunodeficiency virus vaccine development has proven equally difficult, as there is no licensed product after more than 30 years of pre-clinical and clinical research. Here, we examine vaccine development with attention to the host. We discuss how nuclear hormones, including vitamins and sex hormones, can influence responses to vaccines. We show how nuclear hormones interact with regulatory elements of immunoglobulin gene loci and how the deletion of estrogen response elements from gene enhancers will alter patterns of antibody isotype expression. Based on these findings, and findings that nuclear hormone levels are often insufficient or deficient among individuals in both developed and developing countries, we suggest that failed vaccine studies may in some cases reflect weaknesses of the host rather than the product. We encourage analyses of nuclear hormone levels and immunocompetence among study participants in clinical trials to ensure the success of future vaccine programs.

Epitope-Specific Serological Assays for RSV: Conformation Matters

Respiratory syncytial virus (RSV) causes substantial morbidity and mortality in children and older adults. An effective vaccine must elicit neutralizing antibodies targeting the RSV fusion (F) protein, which exists in two major conformations, pre-fusion (pre-F) and post-fusion (post-F). Although 50% of the surface is shared, pre-F contains highly neutralization-sensitive antigenic sites not present on post-F. Recent advancement of several subunit F-based vaccine trials has spurred interest in quantifying and understanding the protective potential of antibodies directed to individual antigenic sites. Monoclonal antibody competition ELISAs are being used to measure these endpoints, but the impact of F conformation and competition from antibodies binding to adjacent antigenic sites has not been thoroughly investigated. Since this information is critical for interpreting clinical trial outcomes and defining serological correlates of protection, we optimized assays to evaluate D25-competing antibodies (DCA) to antigenic site Ø on pre-F, and compared readouts of palivizumab-competing antibodies (PCA) to site II on both pre-F and post-F. We show that antibodies to adjacent antigenic sites can contribute to DCA and PCA readouts, and that cross-competition from non-targeted sites is especially confounding when PCA is measured using a post-F substrate. While measuring DCA and PCA levels may be useful to delineate the role of antibodies targeting the apex and side of the F protein, respectively, the assay limitations and caveats should be considered when conducting immune monitoring during vaccine trials and defining correlates of protection.

Structure basis of neutralization by a novel site II/IV antibody against respiratory syncytial virus fusion protein

Globally, human respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infections in newborns, young children, and the elderly for which there is no vaccine. The RSV fusion (F) glycoprotein is a major target for vaccine development. Here, we describe a novel monoclonal antibody (designated as R4.C6) that recognizes both pre-fusion and post-fusion RSV F, and binds with nanomole affinity to a unique neutralizing site comprised of antigenic sites II and IV on the globular head. A 3.9 Å-resolution structure of RSV F-R4.C6 Fab complex was obtained by single particle cryo-electron microscopy and 3D reconstruction. The structure unraveled detailed interactions of R4.C6 with antigenic site II on one protomer and site IV on a neighboring protomer of post-fusion RSV F protein. These findings significantly further our understanding of the antigenic complexity of the F protein and provide new insights into RSV vaccine design.